Dimethyl terephthalate is widely utilized in the manufacture of polyethylene terephthalate, a plastic employed in plastic beverage containers, Dacron.TM. clothing material and other commercial products. In the manufacturing process, dimethyl terephthalate typically is mixed with ethylene glycol and brought into contact with an acidic catalyst, such as an oxide of antimony. A substantial portion of the dimethyl terephthalate reacts with the ethylene glycol to produce polyethylene terephthalate, methanol, and water. Some intermediate polymers, such as oligomers of dimethyl terephthalate are also produced.
Polyethylene terephthalate and other relatively volatile components are typically recovered by distillation in a resin tower. As a by-product, the distillation produces a bottoms stream which includes unreacted dimethyl terephthalate, relatively low molecular weight dimethyl terephthalate oligomers, ethylene glycol, and water. Additionally, the bottoms stream often contains a small amount of residual polyethylene terephthalate, traces of the acidic catalyst, and insoluble contaminants. Corrosion products and relatively high molecular weight polymers produced by thermal degradation are examples of such insoluble contaminants.
Previously, unreacted dimethyl terephthalate which entered the bottoms stream has been considered unrecoverable. The bottoms stream is not susceptible to conventional filtration because the mixture of dimethyl terephthalate, ethylene glycol, and water quickly fouls conventional filters. Somewhat longer filter runs can be obtained by applying a filter precoat, such as a coating of diatomaceous earth, on the conventional filters, but such precoats tend to become mixed with the filter cake. Additionally, conventional filtration yields a filter cake of dimethyl terephthalate that is contaminated by catalyst, corrosion products, and thermally degraded polymer products.
Upon leaving the resin tower, the bottoms stream is usually subjected to evaporation, stripping, or distillation steps which reclaim ethylene glycol for recycling and which produce a relatively more concentrated residual bottoms stream. Unfortunately, as the level of ethylene glycol in the residual bottoms stream is reduced, the dimethyl terephthalate and its oligomers become increasingly heat sensitive. Distilling the residual bottoms stream to about 10% to about 15% ethylene glycol content by volume thermally degrades polymers present in the stream to such an extent that the stream exhibits a viscosity similar to modeling clay. For example, the residual bottoms stream is sometimes transported and stored in the form of massive blocks.
Consequently, the residual bottoms stream produced as a by-product of dimethyl terephthalate manufacturing has long been considered worthless. After the residual bottoms stream has been concentrated by removing as much ethylene glycol as is practical, it is commonly incinerated as hazardous waste at significant cost. Alternatively, the residual bottoms stream may be placed in a landfill. Ethylene glycol which remains in the residual bottoms stream is susceptible to leeching and can contaminate groundwater if it escapes the landfill. Regrettably, the residual bottoms stream is a poor candidate for subsequent size reduction processes which might otherwise facilitate disposal or re-use, because the stream is usually a liquid or an extrudable solid which resists pulverizing and grinding.
A need exists for a practical method for separating dimethyl terephthalate polymers from ethylene glycol. Such a recovery process would yield valuable ethylene glycol, and would also prevent unnecessary filling of increasingly scarce landfills.